In recent years, bioplastics formed from natural plants as a raw material have been receiving attention in view of the global warming issue. Bioplastics are formed from plants that are originally present on earth, and thus the major advantage of bioplastics is the carbon neutral property thereof, i.e., they exerts no influence on increase and decrease of carbon dioxide on earth. Bioplastics are produced by using biomass-derived materials, i.e., substances that are formed by plants through fixation of carbon dioxide in the air, and therefore, disposal by combustion thereof provides even balance for carbon dioxide, which may be measures against global warming.
Polylactic acid is a polymer formed by polymerizing lactic acid through ester bond, and is one of bioplastics capable of being synthesized from plant-derived materials. On acting lactobacillus on glucose, sucrose or the like, lactic acid is obtained by the fermentative action thereof. The saccharides as raw materials may be mass-produced by acting an enzyme (such as amylase) on starch obtained from potatoes, corns or the like or by extracting from sugarcanes. The demand of polylactic acid has been increased in recent years from the standpoint of “carbon neutral”.
However, corns and sugarcanes as raw materials of polylactic acid are also utilized as foods, and thus there may be an issue of competition from foods.
Lactic acid has one asymmetric carbon atom and includes two optical isomers, i.e., an L-isomer and a D-isomer. A polymer obtained by polymerizing only the L-isomer is referred to as poly-L-lactic acid, and a polymer obtained by polymerizing only the D-isomer is referred to as poly-D-lactic acid. It has been known that these polymers have helical structures that are inverse to each other due to their respective configuration, and it has been said that the melting point thereof is approximately 175° C.
On the other hand, it has also been known that a mixture of poly-L-lactic acid and poly-D-lactic acid is a resin having higher heat-resisting property due to formation of an eutectic crystal of poly-L-lactic acid and poly-D-lactic acid than poly-L-lactic acid or poly-D-lactic acid. The resin is referred to as stereocomplex polylactic acid (SC-PLA), and it has been said that the melting point thereof is approximately 225° C.
Direct polymerization of L-lactic acid and D-lactic acid from a mixture thereof forms a polymer that has L-lactic acid and D-lactic acid mixed randomly in one lactic acid polymer chain, i.e., so-called poly-DL-lactic acid. Poly-DL-lactic acid shows no crystallinity and is softened at a glass transition temperature around 50° C., and thus the polymer is not suitable for practical use.
For producing stereocomplex polylactic acid, it has been necessary that poly-L-lactic acid and poly-D-lactic acid are separately produced and then mixed, which is inefficient. A saccharide is converted mainly to L-lactic acid on acting Stc. Thermophilus or the like as an L-lactic acid producing strain among the lactobacillus strains. A saccharide is converted mainly to D-lactic acid on acting Leus. Mesenteroides or the like as a high purity D-lactic acid producing strain among the lactobacillus strains. Accordingly, for producing L-lactic acid and D-lactic acid separately, it is necessary to use separate strains and to perform fermentation separately, which are inefficient.
Other known production methods of lactic acid include a method of forming cyanohydrin from acetaldehyde in the presence of an enzyme, and then acid-hydrolyzing cyanohydrin (see Non-patent Document 1), and a method of forming cyanohydrin from acetaldehyde and hydrocyanic acid, and then esterifying cyanohydrin, for example, in the presence of hydrochloric acid. According to the methods, racemic lactic acid containing equal amounts of L-lactic acid and D-lactic acid mixed is produced.
On polymerizing racemic lactic acid directly, however, only poly-DL-lactic acid containing L-lactic acid and D-lactic acid mixed in one lactic acid polymer chain, but stereocomplex polylactic acid cannot be produced. In order to produce stereocomplex polylactic acid from racemic lactic acid, it is necessary to separate L-lactic acid and D-lactic acid by a method for designing optical resolution. As the method for designing optical resolution of racemic lactic acid, a crystallization method and a separation method by chromatography have been known, but these methods require complicated operations and high cost, and it is difficult to separate a large amount of compounds by the methods.
Furthermore, acetaldehyde is produced almost from petroleum materials, and therefore, polylactic acid formed by polymerizing lactic acid that is produced by the methods may not be considered as bioplastics.    Non-patent Literature 1: Chem. Commun., 2001, p. 1800